Obscuring the ultraviolet reflective markings on male Cosmophasis umbratica jumping spiders messed up how the spiders recognized each other as male or female, researchers report.
The study is the first to investigate the impact of two different types of UV light on sexual signaling; UVA—UV light with wavelength between 315 and 400 nanometers, and UVB—UV light with wavelength between 218 and 315 nanometers. In the past, similar studies have focused only on UVA or UVB.
“Only the male reflects UVA and UVB but the females don’t,” says study leader Li Daiqin, associate professor with NUS Biological Sciences. “It’s a very important phenomenon called sexual dimorphism in colors.”
The markings led the team to hypothesize that the males use the UV reflective markings to attract the females, while the markings on the males act as a factor for the females’ choice of mate and male-male competition.
The researchers used filters that selectively blocked UVA and UVB reflected from the male spiders during the experiments, in which they observed male-male interactions and male-female interactions. They found that female spiders spend significantly more time interacting with males when UVB was not blocked as opposed to males with UVB blocked.
However, when the female spiders faced male spiders with UVA blocked, the former displayed agonistic behavior instead of courtship behavior. This indicates that they no longer perceive the male as a potential mate, but as a rival.
Similarly, male spiders displayed courtship behavior to other male spiders when UVA was blocked. This indicates that due to lack of UVA reflection, the male spiders no longer view the other male as a rival, but as a potential mate.
The researchers were able to conclude that UVB is a factor in mate choice while UVA is important in sex recognition.
These UV reflective markings are structure colors, Li explains. Unlike commonly used chemical colors like paint, the colors are not pigments but instead produced by the arrangement of physical structures like scales and feathers.
Human eyes see them as iridescence and, at different angles, the structures can display different colors. This allows the spider to simultaneously camouflage itself and hide from predators while sending courtship signals. Research on structure colors includes biomimicry for use in the military industry.
The findings of the study appeared in the journal Animal Behaviour earlier this year.
Source: National University of Singapore